Use the J1939 SPN FMI Lookup tool below to instantly find diagnostic information for any fault code. Enter the SPN number, the FMI code, or both together — for example “157 18” or “spn 1569 fmi 31” — and get instant results.
J1939 SPN FMI Lookup: Instant Fault Code Diagnostic Reference
The SAE J1939 diagnostic protocol identifies faults through a combination of two parameters: the Suspect Parameter Number (SPN) and the Failure Mode Identifier (FMI). Together, these two values form a Diagnostic Trouble Code (DTC) that pinpoints exactly which component has failed and in what way. Every ECU-equipped heavy-duty vehicle — from on-highway trucks and semi-trailers to agricultural tractors, construction equipment, and marine vessels — uses this standardized coding system to communicate faults across the J1939 CAN bus network.
The SPN identifies the specific parameter or component involved in the fault. With over 33,000 standardized parameter numbers defined in SAE J1939DA, SPNs cover every measurable signal in a modern heavy-duty vehicle: engine oil pressure, fuel rail pressure, exhaust aftertreatment temperatures, transmission output shaft speed, wheel brake lining wear, and thousands more. Each SPN has a defined unit of measurement, a valid operating range, and a set of applicable FMIs.
The FMI defines the nature of the failure. The 32 standardized failure mode identifiers — ranging from FMI 0 (data above normal range, most severe) through FMI 31 (condition exists) — describe whether the fault is electrical, mechanical, data-related, or a system-level condition. FMI 3 indicates voltage above normal or a short to supply. FMI 4 indicates voltage below normal or a short to ground. FMI 5 points to a current below normal or an open circuit. FMI 7 signals a mechanical system not responding to commands.
J1939 Hub documents every SPN and FMI combination with manufacturer-grade technical content: a precise fault description, common symptoms observed on the vehicle or equipment, probable root causes ranked by frequency, advanced ECM-level analysis covering signal processing logic and safety fallback strategies, and a step-by-step troubleshooting guide based on real workshop scenarios. Coverage spans all major systems and manufacturers including Cummins, Detroit Diesel, PACCAR, Volvo Trucks, Caterpillar, John Deere, and Bosch.